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The rapid expansion of renewable energy installation capacity is gaining significant attention and is projected to increase in the upcoming years due to combating climate change induced by excessive greenhouse gas (mainly CO 2) emissions. However, the mismatch between renewable energy supply and consumption requires efficient electricity storage devices. As the global economy remains dependent on carbon-based chemicals, the application of electrosynthetic processes that convert excess electricity and CO 2 into chemicals and energy-intensive fuels has attracted considerable attention. In this case, a microbial electrolysis cell (MEC) could leverage extracellular electron transfer to promote cellular metabolism, sustainably synthesizing diverse CO 2 -reducing products. Before fulfilling the potential of MEC, a comprehensive understanding of the biological mechanisms behind extracellular electron uptake is required to realize efficient channeling of cathode-derived electrons toward microbiomes for prominent CO 2 valorization. Hence, this review starts with a comprehensive overview of typical electroactive microbiomes involving extracellular electron uptake and their associated genetic evidence. Then, viable methods, including spectroscopy, microscopy, electrochemical, and meta-omic methods, are introduced to facilitate the quantification and qualification of the electron uptake process. Finally, existing strategies for enhancing extracellular electron uptake for a prominent CO 2 bioelectrosynthesis performance are discussed. [Display omitted] • Microbial electrolysis cell (MEC) represents ample strategy for CO 2 valorization. • Sluggish extracellular electron uptake (EEU) constraints MEC performance. • EEU patterns of critical electroactive microbiomes in MEC are elucidated. • Qualitative and quantitative methods are summarized to verify EEU. • Reinforcement strategies for the enhancement of EEU are discussed. [ABSTRACT FROM AUTHOR]

Electromethanogenesis has emerged as a biological branch of Power-to-X technologies that implements methanogenic microorganisms, as an alternative to chemical Power-to-X, to convert electrical power from renewable sources, and CO 2 into methane. Unlike biomethanation processes where CO 2 is converted via exogenously added hydrogen, electromethanogenesis occurs in a bioelectrochemical set-up that combines electrodes and microorganisms. Thereby, mixed, or pure methanogenic cultures catalyze the reduction of CO 2 to methane via reducing equivalents supplied by a cathode. Recent advances in electromethanogenesis have been driven by interdisciplinary research at the intersection of microbiology, electrochemistry, and engineering. Integrating the knowledge acquired from these areas is essential to address the specific challenges presented by this relatively young biotechnology, which include electron transfer limitations, low energy and product efficiencies, and reactor design to enable upscaling. This review approaches electromethanogenesis from a multidisciplinary perspective, putting emphasis on the extracellular electron uptake mechanisms that methanogens use to obtain energy from cathodes, since understanding these mechanisms is key to optimize the electrochemical conditions for the development of these systems. This work summarizes the direct and indirect extracellular electron uptake mechanisms that have been elucidated to date in methanogens, along with the ones that remain unsolved. As the study of microbial corrosion, a similar bioelectrochemical process with Fe0 as electron source, has contributed to elucidate different mechanisms on how methanogens use solid electron donors, insights from both fields, biocorrosion and electromethanogenesis, are combined. Based on the repertoire of mechanisms and their potential to convert CO 2 to methane, we conclude that for future applications, electromethanogenesis should focus on the indirect mechanism with H 2 as intermediary. By summarizing and linking the general aspects and challenges of this process, we hope that this review serves as a guide for researchers working on electromethanogenesis in different areas of expertise to overcome the current limitations and continue with the optimization of this promising interdisciplinary technology. • Electromethanogenesis is a promising technology being optimized at the lab scale. • Interdisciplinary work is required to overcome the system limitations for upscaling. • New mechanisms on electron uptake have been elucidated in methanogens. • Electrochemical parameters must suit the methanogenic electron uptake mechanisms. • Non-electrotrophic methanogens are key biocatalysts for this technology. [ABSTRACT FROM AUTHOR]

• A lithographic strategy was used to induce biofilm formation in a modified Shewanella oneidensis cdrAB strain to investigate the relationship between biofilm thickness and cathodic activity. • Differences in cell density at the electrode interface (controlled by blue light exposure time) resulted in measurable differences in biologically driven cathodic current density. • Electrochemical Impedance Spectroscopy (EIS) demonstrated that differences in impedance corresponded to biofilm thickness and activity. • An equivalent circuit model, that showed less than 5 % error on fitted parameters, was used to calculate the biofilm charge transfer resistance. • A linear relationship between biological cathodic current and biofilm charge transfer resistance was observed with correlation score r = 0.87 (r2 = 0.773). A range of biotechnological applications for converting electricity to fixed metabolic substrates is fuelling the study of cathodic bioelectrochemical systems. Shewanella oneidensis MR-1 has emerged as an important model system for extracellular electron uptake on cathodes, as many of the proteins involved in this process overlap with the organism's previously characterized extracellular electron transport machinery. However, there are still many questions surrounding the mechanics of electron uptake in Shewanella stemming largely from the challenge of quantifying biomass on electrode surfaces. This limits our understanding of the physiologic and kinetic constraints of electron uptake, as well as our ability to make meaningful comparisons across systems. To investigate the relationship between cathodic activity and biomass, we used a Shewanella oneidensis strain genetically modified with cell aggregation protein CdrAB behind a blue light-controlled promotor. Using blue light exposure to control cell deposition, we then investigated the relationship between cathodic activity and biomass. Electrochemical impedance spectroscopy (EIS) confirmed a decrease in biofilm impedance over a range of blue light exposures (i.e., 2 to 8 h). Consistent with previous results, after this timepoint, a drop-in electrochemical activity was observed, and impedances increased. For biofilms within the 2–8 h light exposure range, we observed a trend towards increased biological current consumption by quantifying the difference between pre and post kill currents. Comparing EIS data between pre and post kill experiments supported an increase in impedance post addition of killing agents and a trend towards the lowest biofilm impedances observed in the longest blue light exposed systems. Using an equivalent circuit model to extrapolate specific biofilm parameters we quantified the charger transfer resistance within the biofilm that corresponds to varying biofilm thicknesses and matches previously observed activities. For example, electrochemical activity was highest for the 8 h blue light exposed biofilm condition, with a maximum cathodic biologic current of -5.49 ± 0.85 µA, and a biofilm charge transfer resistance measured at 6909.5 ± 2136.5 Ω. On an individual reactor basis, we correlate this biofilm charge transfer resistance with biologic cathodic current. We observed a linear trend with a correlation score of 0.87 (r2 = 0.773). To the best of our knowledge, this is the first investigation of biofilm physiology on Shewanella cathodes using EIS. Continued efforts in this direction will further our understanding of biofilm-electrode interface during extracellular electron uptake with the goal of enhancing applications to bioelectrochemical systems. [Display omitted] [ABSTRACT FROM AUTHOR]

• Dual-electric centers catalysts cooperate with microbes to effectively remove CIP. • Delocalized electrons of absorbed CIP captured by microbes at the electron-rich area. • The catalysts synergize extracellular electron transfer for the cleavage of CIP. • The accumulation of antibiotic resistance genes was reduced by the catalysts. Antibiotics and antibiotic resistance genes (ARGs) are still a problem in biological treatment. Herein, we propose a synergetic strategy between microbes and dual-electric centers catalysts (CCN/Cu-Al 2 O 3 /ceramsite) for Ciprofloxacin (CIP)-contained (5 mg/L) water treatment in an up-flow biological filter. CIP was cleaved into small molecules by the catalyst, bringing a 57.6% removal and reducing 10.5% ARG. The characterization results verified that a Cu-π electrostatic force occurs on the catalyst surface, forming electron-rich areas around Cu and electron-poor areas at the carbon-doped g -C 3 N 4 (CCN) aromatic ring. Thus, the electrons of adsorbed CIP were delocalized and then captured by the adsorbed extracellular polymeric substance at the electron-rich areas. Therefore, the synergetic process weakened the stress of CIP on bacteria and reduced ARG accumulation. It also enriched more electro-active bacteria on the surface of CCN/Cu-Al 2 O 3 /ceramsite, promoting the expression of extracellular electron transfer-related genes and reconstructing the energy metabolism mode. This result provides an opportunity for refractory antibiotic treatment in the biological process. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • In-situ biogas upgrading was achieved in a AD/PSB-coupled MEC. • Photosynthetic CO 2 fixation under electrical stimulation increased by 83.3%. • Electrically-stimulated PSB fixed CO 2 more efficiently even under power off. • Electrical stimulation enhanced the electroactivity and photoactivity of PSB. • Electrical stimulation changed the structural of photosynthetic reaction center. Biogas from anaerobic digestion usually contains 30%–50% of CO 2 , which needs to be upgraded before industrial utilization. However, current upgrading techniques typically consume large amounts of chemicals and energy. This study designed a green and low-energy-consumption microbial electrolysis cell (MEC)-based anaerobic digester for in-situ biogas upgrading, in which the cathode with photosynthetic bacteria (PSB) was used to fix CO 2. Results showed that charged MEC-AD increased photosynthetic CO 2 fixation by 83.3% and CH 4 production by 62.8% compared with uncharged open-circuit MEC-AD. Adding an extracellular electron uptake inhibitor to the cathode decreased the CO 2 fixation, accompanied by the decline of current between the two electrodes, which suggested that the cathode electron uptake was the main reason for the facilitated CO 2 fixation of PSB. The electrically-stimulated PSB fixed CO 2 more efficiently than unstimulated PSB even under power off, likely related to their enhanced electroactivity and photoactivity that could acquire more extracellular electrons and light energy for CO 2 fixation. The carbonyl-related hydrogen bonds in the LH1 complex of photosynthetic reaction centers increased under electrical stimulation, resulting in a red shift of absorbance spectra to improve the energy utilization of PSB. This upgrading technique consumed about 0.37 kWh/Nm3 CO 2removed of electric energy, much lower than other MEC-based techniques, providing a green and feasible strategy for in-situ biogas upgrading. [ABSTRACT FROM AUTHOR]